Admittedly I can only speak to the merits of Kumu. I was never really trained in system dynamics, more systems thinking, so that plus the wild expense of most SD software means that I have never really given them a chance.

The other reason I can't compare them is because once I tried Kumu, I never looked back. Kumu is wonderful. It looks simple enough, but once you dig into the advanced features and the extensive documentation, you can really learn to do amazing things with it. I've used it for many years now for many clients.

But see again point 1. It isn't made to be a systems dynamics modeling tool, so simulation and analysis features you might expect to find might be missing.

My suggestion is to choose a problem to work, create a free model, and start messing around. My guess is that you'll find Kumu great for initial sketches and non-dynamics models, but you'll probably want to go elsewhere for simulation work.